The 40 S&W hit the law enforcement market and took it by storm in 1990. A more powerful round than the 9mm with more mag capacity than the 45 acp in guns the same size as those for the 9mm. The 40 took off and has not really slowed down at all. The law enforcement agencies that had not switched from wheelguns to autos now had a choice of the 9mm or the 40 in higher capacity mags. The U.S. military had just gone to the M9 and more rounds was the way to go.
The 357 Sig showed up 4 years later and late to the show. As was mentioned earlier in this thread, new rounds often have teething pains and the 357 Sig had it's own and that held it back as well. Agencies that were looking for new guns and rounds 1990-1994 had committed to the 9 or the 40 or 45acp and there was little to be had for the Sig round from these agencies. As you can see in this thread adoption by law enforcement guarantees sales in the civilian market. There was little room for the Sig round at that time. The 9 and 40 were known quantities and the 40 pretty new no reason to go with a newer round.
In the early 90s the semis you saw on the hips of LEOS in the U.S. were 3rd Gen S&Ws, the new Beretta 92s, here and there the 1911. The Sig P220 or P229 could be seen and a new gun from Austria that was cheap and controversial and offered great deals to law enforcement and could be had in 9 or "fortay".
Fast forward about 22 years and new guns and rounds dominate the market. About 2-3 years ago Sig launched a new campaign to sell the 357 Sig round to Leos. The round had languished and they hoped to revive interest. Nothing wrong with that. It's a good round and folks won't go wrong getting it.
Power wise (ft. lbs of energy) it falls about the same level as the 155 and 165 gr. loads for the 40 S&W while making a smaller hole and having a lighter bullet and offering the same mag capacity. It can penetrate more 1/8 inch steel plates than the 9 or the 40 with comparable bullets. But after you have penetrated 2-3 such plates does it mean all that much to penetrate 3 and dent the 4th? But it will do it (see the links I provided earlier in this thread). The 9, the 40, the 45acp and the 38 Super all easily penetrate auto and truck bodies and auto glass as well as the .357 Sig.
As for ease of feeding, when was the last time you heard fellas complaining that the 9mm would not feed in their Glock 19 due to the shape of the cartridge?
The Sig round is a good one get it cause you want to try it and keep it cause you like the way it shoots and you handle it well. Beyond that there is no need for hype.